Optical fibers are often used in the gas and oil industries for down-hole monitoring operations to gather and analyse borehole data. The fibers are encased in various designs of cable to ensure mechanical robustness.
Due to the high temperatures, pressures and operation within an atmosphere containing polycarbons in the oil and gas industry, along with the presence of gases and other potentially corrosive conditions in a down-hole environment, there is a need for optical fibers that are resistant to these extreme conditions and that maintain a high level of performance.
In undersea environments optical fibers are used for both sensing and communications and hydrogen is also present.
Attenuation in optical fibers and loss of performance may result from of a number of different factors such as absorption, scattering, bending losses, as well as at fiber connections. Each loss mechanism is relative to the attenuation properties of the optical fiber which are influenced by the fiber material properties and fiber structure.
Absorption is arguably the most significant cause of attenuation and performance loss in an optical fiber. Absorption may be defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. Imperfections in the atomic structure of the optical fiber induce absorption by the presence of vacancy defects or oxygen defects (point defects). Silica fibers are commonly used because of their low intrinsic material absorption at the wavelengths of operation in optical fibers.
Absorption is also induced by the diffusion of hydrogen molecules into the glass fiber. The absorption of hydrogen into silica glass fiber has two mechanisms.
The first absorption mechanism is by temporary diffusion wherein hydrogen diffuses into interstitial sites in the silica network of the glass fiber. This phenomenon has been found to affect all silica-based glass fibres, both single and multimode.
The most significant effect of temporary hydrogen diffusion is a strong absorption at 1.24 μm. Further peaks are also observed at longer wavelengths (a rising loss edge beyond about 1.5 μm). In practice, the diffusion of hydrogen into the fibre core can be measured by the growth of the 1.24 μm peak. As the concentration of hydrogen increases, the breadth of each absorption band also increases. Overall, these absorptions prevent the transmission of light along the optical fiber at the required wavelength.
The solubility of hydrogen in bulk silica is linear with pressure; however, it decreases as temperature increases. It should be noted that the saturation level of hydrogen that can be absorbed into an optical fibre is greater than that required to optically darken the fibre and thus the effect of temperature is not significant.
The losses due to hydrogen absorption caused by the temporary diffusion mechanism are reversible. When external pressure and/or the hydrogen source is removed, the absorbed hydrogen diffuses out of the core and the light transmission characteristics are restored with time.
The second mechanism for absorption of hydrogen into silica glass fiber is a chemically induced absorption mechanism which results from chemical reaction of the diffused hydrogen with the silica glass constituents to form, for example, hydroxyl (OH) groups. These hydroxyl groups have distinctive absorption bands that lead to high attenuation of the transmitted light, a phenomenon known as ‘hydrogen darkening’.
The intensity of all chemically induced absorptions increases with time as the chemical reactions proceed. The rates of reaction are also accelerated at higher temperatures (such as in down-hole environments) and at increased hydrogen concentrations. The chemical reactions are irreversible and thus once it has been created, chemically induced attenuation cannot be removed. Some examples of chemically induced absorption of hydrogen that lead to hydrogen darkening in silica fibers will now be described.
Reaction with Silica
Even for a pure silica fibre, the silica within the fibre core does not comprise a pure silicon-oxygen three-dimensional matrix (—Si—O—Si—O—). There remain some terminal sites where oxygen forms a double covalent bond to silicon (Si═O), especially at the edges of the fiber core. Under certain conditions, these can react with hydrogen to produce terminal ‘dangling’ hydroxyl groups (Si—OH). Alternatively, hydrogen can react with non-bridging (radical) oxygen hole centres (Si—O.) or peroxy radicals (—Si—O—O.) to produce OH groups. Hydroxyl groups absorb strongly in the transmission region of interest 1.38, 0.95 and 0.72 μm, the strongest absorption being 1.38 μm. There is also an absorption at 1.24 μm but this is often masked by the larger peak resulting from molecular hydrogen (described above).
Reaction with Dopants
Silica fibers are commonly doped with germanium (Ge), phosphorus (P) or other Group III or Group V elements. As well as reacting with the silica core of the fiber, hydrogen can also chemically react with these dopants. This can lead, for example, to the formation of Ge—OH sites which absorb strongly at around 1.4 μm. Ge-doped fibres are more susceptible to hydrogen darkening than pure silica core fibres and chemically induced attenuation is worse when both GeO2 and P2O5 are present in the core of the fibre as the P2O5 catalyses the reaction with hydrogen to produce terminal germanium hydroxyl (Ge—OH) species. Additionally, reaction of hydrogen with GeO2-doped fibres can also induce attenuation in the shorter wavelength region (<1 μm).
Essentially, doped fibres are not suitable for prolonged use in extreme environments, even when they have further protection from cable coating or housing.
Other Reactions
Hydrogen can also react with defects within the fiber core that contain alkali impurities (e.g. Na, Li, K, etc.). This is manifest in the growth of a long wavelength absorption edge. Additionally, hydrogen can induce attenuation in undoped fibres (pure silica cores), including those which have a fluorine-doped cladding. In this case, absorptions due to HF (1.44 μm) and SiH (1.53 μm) can be detected.
Reducing Hydrogen Attenuation Losses
Several methods to prevent hydrogen reaching the core of a silica fiber are known. Carbon coating is a well-established technique, for example, U.S. Pat. No. 5,000,541 discloses a method of applying an outer carbonaceous coating between the glass and the protective ‘plastic’ captures. However, ambient temperatures in gas and oil industry environments such as in down-hole operations are often well above the temperature at which an external coat of carbon prevents hydrogen ingress. The effectiveness of carbon coating decreases at temperatures above 170° C. because hydrogen molecules and atoms are very small, their mobility and diffusion increases significantly with temperature and the carbon becomes increasingly porous. There is some residual effectiveness at a typical operating temperature of 300° C. but enough hydrogen diffuses through to ensure the fiber darkens due to the chemical reactions outlined.
Internal protection of the core with additives is also known in the art. It is known to achieve fiber passivation by treating drawn fibers with deuterium at temperature. The technique is described in the ‘Speciality Optical Fibers Handbook’ by Alexis Mendez and T. T. Morse, Academic Press (2007). Deuterium competes with hydrogen in reactions to form Si-OD or Si-D species whose absorption losses occur at much longer wavelengths outside normal operating ranges (>1625 nm) and thus are harmless. However, deuterium treatment is known to be expensive, potentially hazardous during fabrication, and difficult due to the post-process of passivating with deuterium is to use temperature to ensure the deuterium diffuses into the fibre. Hence the exposure time to deuterium has to be long, to ensure adequate diffusion and saturation at the maximum temperature that the fibre can withstand.
Hydrogen (H2) Concentration in Operational Environments such as in Oil and Gas Industries
Hydrogen is present in an oil and gas industries because of the materials used as well as the inherent environment.
Hydrogen exists in the atmosphere, but at less than 1 ppm by volume. In an oil/gas environment (for example, a down-hole environment), the natural concentration of hydrogen may be significantly greater but will still be a relatively minor component.
Hence it is normal practise to test for hydrogen induced darkening in the range between 1 atmosphere (1 bar) pressure of hydrogen as a minimum and up to 15 atmospheres of hydrogen. Although possibly an excessive test, the pressure in an oil well is high and the concentration of hydrogen is not always known before the hydrogen darkens the fiber.
Hydrogen can also be formed by the electrolytic reaction between metallic elements in cables, by water corrosion with metal components, and by the evolution of gasses from decomposition or chemical reactions of fiber coating materials. The most significant of these can be through an electrolytic reaction between different metals placed in a down-hole environment. This is a factor that is not always appreciated when the choice of materials is being made. A poor choice, combined with the presence of aqueous-based production or environmental fluids, may lead to generation of a galvanic cell and consequent production of significant levels of hydrogen.
Hydrogen sulphide (H2S) is commonly found in oil/gas environments and can cause significant problems. For example, H2S in water is only mildly acidic but it will reduce iron causing deterioration. H2S may also react with polymers used on optical fiber claddings to weaken/degrade them, although normally not rapidly. In an electrically conductive solution, such as sea water, this reaction is enhanced.
H2S is also the most common and potentially troublesome source of hydrogen. At atmospheric pressure H2S reacts with ‘steel’ to produce molecular hydrogen. H2S also hinders the combination reaction of atomic hydrogen to molecular H2 and thus the molecular hydrogen is able to start to diffuse rapidly into the steel.
In undersea environments the hydrogen is created by similar mechanisms of electrolysis, plus bacterial and algae reactions with any polymer in the presence of the sea water, in addition to the natural density of dissolved hydrogen. The pressure within the cable will be a function of the depth of the water.
There is a need to provide an optical fiber that exhibits a reduction in hydrogen induced attenuation losses (hydrogen darkening) over operating and transmitting wavelengths that can be used for prolonged periods of time in extreme conditions.
In more benign conditions recently developed low water peak (LWP) and zero water peak (ZWP) fibers are prone to react with trace levels of hydrogen, which generates a water peak which the LWP and ZWP designs intend to avoid. This invention also been developed to passivate against this effect.
We have now devised an improved optical fiber package and design that alleviates some of the above-mentioned problems.